This invention relates to a liquid fuel injection nozzle for supplying fuel to an internal combustion engine, the nozzle being of the so-called inwardly opening type and comprising a nozzle body, a blind bore formed in the body and extending from one end thereof, a seating defined at the blind end of the bore, a fuel inlet passage communicating with the bore intermediate the ends thereof, said fuel inlet passage being connected in use to a fuel injection pump, a sleeve slidable in the end portion of the bore remote from the blind end thereof, a valve member slidable within the sleeve, the valve member being shaped for engagement with the seating to prevent fuel flow through an outlet and a spring abutment engaging with the valve member and engageable by the sleeve or a part associated therewith, the arrangement being such that when fuel under pressure is supplied through said inlet passage, the fuel pressure acting on the valve member and the sleeve will effect movement of the valve member and sleeve against the action of a spring which engages said abutment to allow fuel flow through the outlet, the nozzle also including means defining a stop surface engageable by the sleeve or said part to limit the movement of the sleeve, the further movement of the valve member away from the seating being under the action of fuel under pressure acting on the valve member alone.
Fuel injection nozzles of the aforesaid kind provide for two stages of lift of the valve member away from the seating using a single spring. The first stage of movement allows a restricted rate of fuel flow to the associated engine. The nozzle opening pressure is the pressure at which the valve member starts to lift from the seating. The checking of this pressure is required following assembly of the nozzle and it may also be required during the life of the nozzle. The nozzle opening pressure is usually checked by connecting the inlet of the nozzle to the outlet of a hand operated pump with which there is associated a pressure gauge. It is found that if the hand operated pump is operated quickly a reliable indication of the nozzle opening pressure can be obtained. However, operating the pump more slowly it is sometimes found that the pressure required to lift the valve member from the seating is higher.
An investigation of the problem has led to the discovery that when the sleeve or the aforesaid part are urged by the fuel pressure into contact with the spring abutment, an annular seal or seals is/are obtained which prevent fuel which leaks along the working clearance between the sleeve and the valve member from escaping to the usual drain. As a result a pressure build up occurs which can cause the valve member to remain in contact with the seating.